Help with violence against family members

Hi, we've got a 6 year old son who is in the process of being diagnosed with what we strongly suspect is ASD.  The biggest issue we have right now is when he becomes violent and aggressive towards us and his sister.  This happens during meltdowns but also on other occasions such as simply not getting what he wants can trigger an attack when he was happy up until that point.  An example is that he ask to play football with me when I had finished work for the day and I said I would in 2 minutes as I need to get a drink - that prompted an attack.

It used to only happen at home, but if becoming more frequent when we're out.  It's getting to the point that we simply don't feel that we can all go out together, and can only take our children out separately.

We do our best to reduce his anxiety and keep things calm at home and when we go out, like going to the playground early when there is no-one else there.  But things feel like they are getting worse.

If anyone has any advice with what we can do to help prevent these attacks then it would be very gratefully received.  I understand that the best way is to try and prevent them in the first place, but if he seems happy and then can turn in an instant then it's harder to prevent - we can't always give exactly what he want, when he wants.

Thanks